A study of the purinoceptors mediating contraction in the rat colon

Abstract

1 The effects of a number of purine analogues were examined on the rat isolated colon muscularis mucosae. Adenosine, adenosine 5′-monophosphate (AMP), adenosine 5′-diphosphate (ADP), adenosine 5′-triphosphate (ATP), 2-methylthioATP (MeSATP), adenosine 5′-(2-fluorodiphosphate) (ADPβF), adenosine 5′-(β,γ-methylene)triphosphonate (AM PPCP) and adenosine 5′-(α,β-methylene)triphosphonate (AMPCPP) each contracted the muscularis mucosae in the concentration range 1–100 μm. 2 MeSATP was the most potent purine agonist, with a threshold concentration for contraction of 0.05 μm and an EC₅₀ of approximately 0.3 μm, and AMPCPP was less potent than ATP. The enantiomer of AMPPCP, l-AMPPCP, was inactive at concentrations up to 100 μm. 3 The adenosine receptor antagonist 8-(p-sulphophenyl)theophylline (8-SPT, 50 μm) produced approximately 50 fold shifts of the dose-response curves to adenosine, AMP and AMPPCP, whereas those to ATP, MeSATP and substance P (SP) were unaffected. Intermediate shifts were observed for the dose-response curves to ADP, AD$1βF and AMPCPP. With a lower concentration of 8-SPT (10 μm) a dose ratio of approximately 11 was observed for the inhibition of the effects of both adenosine and AMPPCP. 4 ATP was rapidly degraded by the tissue to ADP, AMP and adenosine, AD$1βF was more slowly degraded to AMP and adenosine, and no significant degradation of AMPPCP was detected during 20 min incubation. 5 The results are consistent with the existence in the rat colon muscularis mucosae of a mixed population of purine receptors of P_2Y and P₁ types. The colon thus contains the first documented incidence of a P_2Y-receptor mediating contraction. The powerful inhibition by the P₁-purinoceptor antagonist 8-SPT of the effects of AMPPCP suggests that its action in this tissue is mediated by P₁-purinoceptors, although 8-SPT was more potent here than has previously been demonstrated.